The present invention relates generally to electrolytic cells, and particularly cells where chlorine gas is reduced at the cathode electrode and chloride ions are oxidized at the anode electrode.
One application for such a cell, also referred to as chlorine-chlorine cell, is the separation of chlorine gas from a stream of chlorine and foreign gases. Such foreign gases could include, but are not limited to, carbon dioxide, oxygen and hydrogen gases. Although the chlorine-chlorine cell separation technique could be useful in the manufacture of chlorine gas, the principal application herein relates to zinc-halogen batteries such as a zinc chlorine battery. In the zinc-chlorine battery application, the foreign gases are also referred to as inert gases. This is because these gases are inert in the hydrate formation process whereby chlorine is stored in the battery. During the charging of a zinc-chlorine battery, chlorine gas is evolved at the positive electrode (anode) and zinc metal is deposited on the negative electrode (cathode). Thus, inside the battery casing, the environment is necessarily a chlorine gas environment. However, small quantities of other gases may also be present inside the battery case. For instance, carbon dioxide is evolved during normal operation of the battery as a by-product of the oxidation of the battery graphite. The volumetric rate of carbon dioxide evolution during battery charging is approximately 0.02% to 0.04% of the chlorine gas evolution rate. Consequently, if the carbon dioxide is not purged from the battery system, it will accumulate over a period of charge/discharge cycles, and eventually interfere with the normal operation of the battery. A brief discussion of a portion of the subject matter of the present application and the zinc-chlorine battery application may be found in: Development of the Zinc-Chlorine Battery for Utility Applications, Interim Report, April 1979, pages 36-9, 12, published by the Electric Power Research Institute, Palo Alto, Calif., and is herein incorporated by reference. A discussion of related electrolytic cells may also be found in a co-filed U.S. patent application entitled "Inert Gas Rejection Device For Zinc-Halogen Battery Systems," assigned to the assignee of the present invention, and is herein incorporated by reference.
The present invention provides a novel electrolytic cell for separating foreign gases from a stream of chlorine and foreign gases. Particularly, the electrolytic cell is generally comprised of a cathode electrode for electrochemically reducing chlorine gas into chloride ions, an anode electrode for oxidizing the chloride ions into chlorine gas, a membrane interposed between the anode and cathode electrodes for preventing the transfer of foreign gases to the anode electrode, a housing for aligning the membrane and electrodes in the cell, an aqueous electrolyte contained in the housing, and a power supply for providing a sufficient potential difference across the anode and cathode electrodes to cause the chlorine gas reduction and chloride ion oxidation reactions. The housing also includes a separate outlet on each side of the membrane to vent the foreign gases (cathode side) and chlorine gas (anode side) from the cell.
The present invention further provides for a novel multiple cell system for use when the gas flow rate into one cell is beyond its capacity to reduce all of the chlorine gas entering the cell. Generally, when the chlorine and foreign gas flow rate into a cell is very low, even an inefficient cell will be capable of reducing all or substantially all of the chlorine gas at the cathode. This is especially true if the applied voltage across the cell is relatively high (i.e. about two volts), as it will keep the cathode very cathodic. However, when the gas flow rate is increased significantly, even an efficient cell may not be capable of reducing all of the chlorine gas. This results in unreacted chlorine gas being vented from the cathode assembly along with the foreign gases. This result is unacceptable because it is desirable to vent the foreign gases into the atmosphere. Thus, with relatively high gas flow rates it is a practical necessity to have more than one cell in order to handle any overflow of unreacted chlorine gas from the cell. The subsequent cell would use as its input the outlet from the cathode section of the previous cell. Alternatively, a plurality of anodes and cathodes could be provided in a common housing, where the stream of chlorine and foreign gases would be divided among the number of cathodes to achieve an effective reduction of the gas flow rate in the multiple cell system.
Other features and advantages of the invention will become apparent in view of the drawings and the following detailed description of the preferred embodiments.